Honeycomb having laminates of unidirectional strands

ABSTRACT

A honeycomb core constructed of a plurality of generally sinusoidally shaped ribbons which are bonded together to define honeycomb cells. Each ribbon includes at least one layer of adjacent, unidirectionally oriented strands which are bonded together.

United States Patent Jackson et a1.

[ 1 June 27, 1972 15 HONEYCOMB HAVING LAMINATES OF UNIDIRECTIONALSTRANDS [72] Inventors: Wendell T. Jackson, Walnut Creek; leverley R.Garrett, Moraga; Earl C. Vicars,

Orinda, all of Calif.

[73] Assignee: l'lexcel Corporation, Dublin, Calif. 22 Filed: April 7,1969 211 App]. No.: 14,021

52 u.s.c1. ..161/6s,161/143 51 1111.0 ..B32b3/12 5s FleldoiSearch..16l/57,58,59,68,69,88, 161/89, 142, 143, 221, DIG. 4, 402, 60;244/123;

[56] References Cited UNITED STATES PATENTS FOREIGN PATENTS ORAPPLICATIONS 845,744 8/1960 01am Britain 161/143 Boggs ..'..l61/143 XNebesar 161/60 X Primary Examiner-Robert F. Burnett AssistantExaminer-Henry F. Epstein Attorney-Townsend and Townsend 57 ABSTRACT Ahoneycomb core constructed of a plurality of generally sinusoidallyshaped ribbons which are bonded together to define honeycomb cells. Eachribbon includes at least one layer of adjacent, unidirectionallyoriented strands which are bonded together.

5 Claims, 10 Drawing Figures P'ATE'N'TEDJUHN I972 3.673.058

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INVENTORS WENDELL I JACKSON 2 BY BEVERLEYRGARRETT 28 EARL c. VICARSATTORNEYS HONEYCOMB HAVING LAMINATFS OF UNIDIRECTIONAL STRANDSBACKGROUND OF THE INVENTION Today honeycomb is in extensive use for alarge number of I applications. Depending upon the end use of thehoneycomb it is constructed of various materials such as metallic foil,impregnated paper, or a plastic or resinous material reinforced withstrands such as yarns.

Reinforced plastichoneycomb as such is known. In the past, reinforcedplastic honeycomb was fabricated from one or more layers of a wovenfabric bonded together with a resinous or plastic material.Suchhoneycomb cores have a number of uses and desirable features, notthe least of which are lightweight and low cost. 1

Although such reinforced plastic honeycomb cores are entirelysatisfactory-for many applications, today's increasing stressrequirements coupled with the necessity for low weight have made suchreinforced honeycomb unusable for certain applications. The woven fabricemployed in the cores is an array of a large number of undulated yarnsrigidified by the bonding material. When the core. is subjected toexternal forces, the undulations over the length of the yarn causesubstantial bending moments over the various longitudinal segments ofthe yarns. This increases the stresses to which such forces subject eachyarn and, consequently, reduce the overall strength as well ascompressive modulus of the ribbon and the honeycomb core.

To provide prior art honeycomb cores with the strength required inmanyof todays applications, the ribbons of the core have to be appreciablythickened to prevent their failure. This in turn results-in a weightpenalty which is often unacceptable or, at least, highly undesirable.

SUMMARY OF THE INVENTION The present invention provides a honeycomb coreconstructed of a plurality of generally sinusoidally formed ribbonsbonded together at adjacent nodes to define cell openings. Each ribbonincludes a layer of unidirectionally oriented, adjacent strands whichare bonded together.

The strands are maintained straight over their length except to theextent they follow the configuration of the ribbon. As compared to priorart, woven fabric plastic reinforced honeycomb cores constructedaccording to this invention eliminate the disadvantages from undulatedstrands. Thus, the strands can be stressed to the fullest extent withoutsubjecting them to undesirable bending moments so that thestrength-toweight ratio is' substantially enhanced over that heretoforepossible. i-lon'eycornbs constructed in accordance with thisinvention'have been found to exhibit up to 40 percent greater strengthand up to 100 percent greater shear moduli.

Accordingly, with honeycomb cores of the present invention it ispossible to subject a core of a given weight to substantially higherstresses. The usefulness of honeycomb core in a host of applications,particularly in the aerospace industry, has

thereby been substantially increased.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features, andadvantages of the present invention will be more apparent afterreferring to the following specification and attached drawings in which:

FIG. l is an elevational view of a honeycomb core constructed accordingto the present invention;

FIG. 2 is a fragmentary plan view of the core illustrated in FIG. 1; 7

FIG. 3 is a fragmentary, enlarged plan view of a honeycomb core ribbonconstructed according to the present invention;

FIGS. 4-6 are schematic representations of ribbons having two layers ofunidirectional strands in different relative angular positions; and

FIGS. 7-10 are schematic representations of ribbons similar to thoseillustrated in FIGS. 4'6 but show the use of unidirectional strandlayers in combination with conventional fabric layers.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, ahoneycomb core 12 is constructed of a plurality of longitudinal ribbons14 which have a generally sinusoidal shape as is well known in the art.Adjacent ribbons are bonded together at nodes 16 whereby they definehoneycomb cells 18 which have an axis transverse to the faces 20 of thecore. Any desirable number of ribbons are so bonded together toconstruct honeycomb cores of various sizes. To use the cores they aremachined, cut, shaped, etc., to give them their final configuration.

The ribbons can be constructed from a large variety of materials such asmetallic foil, paper, or reinforced plastic. Prior art reinforcedplastic ribbons generally comprised one or more laminates of a wovenfabric filled withv a resinous or plastic material. In ribbons having aplurality of such laminates the laminates are also bonded together.

FIG. 3 illustrates an enlarged ribbon 14 which is constructed of aplurality of laminates 22-bonded together by a plastic or resinousmaterial. In a presently preferred embodiment of this invention eachlaminate comprises a layer 24 of non-woven strands or fabric yarns '26which lie sideby side, have a unidirectional orientation, i.e., areparallel, and are bonded together by the resinous or plastic material.

Referring to FIGS. 4-6, the orientation of yarns 26 in a ribbonconstructed of two laminates isschematically illustrated. If the ribbonis assumed to longitudinally extendfrom the left to the right in theFIGS., FIG. 4 illustrates a ribbon in which the yarns in the firstlaminates are parallel to the longitudinal extent of the ribbon, andthereby are transverse to the axis of cells 18,'and the yarns in thesecond laminate are transverse to the-longitudinal extent, or parallelto cells 18. FIG. 5 illustrates a ribbon in which the angulardisposition of the yarns in the two laminates is again about butwherein'the yarns in both laminates have an angular inclination to thelongitudinal extent of the ribbon of about 45. In FIG. 6 the angulardisposition of the unidirectionally oriented yarns in the two laminatesdiffers by less than 90. Moreover, the laminates are angularly inclinedto the longitudinal extent of the ribbon.

The preceding illustration of possible variations in the relativepositioning of the yarn layers in each laminate and of the relativeangular disposition of the in the layers makes it clear that furthercombinations and variations can be achieved if desired for theparticular application of a honeycomb core. The number of laminates ineach ribbon can be varied. Thus, in applications requiring lesserstrength of the honeycomb only one laminate may be provided for eachribbon while in high strength applications the number of laminates maybe increased to three or more laminates per ribbon.

Referring to FIGS. 7-10, in another embodiment of this invention eachribbon 14 is constructed of a plurality of laminates 22 in which atleast one of the laminates has a layer of adjacent, unidirectionallyoriented non-woven yarns 26 which are bonded together by a resin or aplastic as described above. At least a second layer 24 of the ribbon isconstructed of a conventionally woven fabric; 28, the strands of whichare likewise bonded together by a resin or a plastic. The laminates arefurther bonded together to fonn the ribbon. The woven fabric layer inthis type of ribbon acts as a base and filler for the unidirectionalyarn layers. The advantages of a significantly improvedstrength-to-weight ratio and shear modulus is obtained while the costsof the ribbon are kept relatively low.

Various combinations in the arrangement of the unidirectional yarn andwoven fabric layers can be made. In FIG. 10a fabric layer and aunidirectional yarn layer are provided to form a two-laminate ribbon.Although ribbons constructed as illustrated in FIG. 10 are entirelysatisfactory, it is preferred that the outermost layers of the ribbonsare identically constructed layers, that is that they are either fabricor unidirectional yarn layers as illustrated in FIGS. 7-9. Warpage andtwisting of the ribbon, and the sheet from which the ribbon is out arethereby prevented. The number of laminates in each ribbon can, ofcourse, be varied over that illustrated in FIGS. 7-9.

In a further embodiment of this invention the fabric layers 28illustrated in FlGS. 7-10 can be replaced by layers of other materialssuch as metallic foils, film, papers, etc. Though each influences thecharacteristics of the resulting ribbon and honeycomb core differently,the benefits of the present invention from employing unidirectionallyoriented yarn layers are obtained, and combined with the particularcharacteristics obtained from the use of such materials in honeycomb.

The material for constructing yarns 26 can be chosen according to thedesired ultimate characteristics of the honeycomb core. Thus, they maybe constructed of glass fibers for low-cost cores, of carbon or graphiteyarns for high strength applications, of asbestos or polyimide fibersfor high temperature applications, etc. Similarly, the material forbonding the strands of yarn to each other can be chosen from a broadclass including thermoplastic or thermosetting plastics, resins,polyimides (for high temperature application) etc. All these materialshave the common characteristic that they can be applied to the strandsin an essentially liquid form and can thereafter be hardened to form inconjunction with the yarns, a sheet or laminate that can be handled tofabricate ribbons, give them their-sinusoidal shape, and to bond themtogether at the nodes to form honeycomb cores.

. Strength and stress considerations make it sometimes desirableto'employ ribbons having either only one laminate or laminates in whichthe yarns are parallel or so positioned that they can warp the ultimateribbon and panel from which the ribbon is cut. Such warpage can beprevented by control of the twist direction in the yarns. Yarns aregenerally twisted in either a clockwise or counterclockwise direction,or as more commonly known in either the Z or the 8 direction. This twistin the yarns is responsible for warpage of the ribbons. If the ribboncomprises only a single laminate the twist of the yarns in theunidirectionally oriented yarn layer is alternated, a Z following an Stwisted yarn or a plurality of Z twisted yarns following a plurality ofS twisted yarns. If the ribbon is constructed of two laminates in whichthe yarns in the two layers are parallel the yarns in each layer areconstructed of oppositely twisted yarns. warpage of the ribbons andpanels for which they are constructed is thereby prevented.

Turning to the fabrication of ribbons, it is presently preferred to wrapyarn about an elongated cylinder or mandrel so that the individual yarnslay side-by-side, tough each other and are oriented in the samedirection. Thereafter, the selected resin or plastic is applied to thelayer and is permitted to at least partially dry to impart some rigidityto the laminates. If several layers are required to form the ribbon,additional yarn layers are applied over the firm one or a plurality ofsingle, bonded yarn layers are subsequently bonded together to form amultilaminate ribbon. The orientation of the yarns is controlled toobtain the desired angular disposition between them and the variouslaminates.

After the resin or plastic has been at least partially dried, theresulting reinforced plastic cylinder is longitudinally slitted to formtwo halves, is removed from the mandrel and is flattened. To aid theremoval of the reinforced plastic semicylinders from the mandrel, thelatter preferably includes a nonstick coating such as teflon or analuminum foil wrapped around the mandrel. I

The flattened reinforced plastic panels are now cut into longitudinalstrips, with the direction of the cuts being controlled by the angulardisposition of the yarns in the panel and the required angulardisposition of the yarns in the ribbon. The ribbons are then corrugatedaccording to well-known methods and bonded together at the nodes withbonding agents, by heat sealing the ribbons to each other, or by curingthem.

The strength of honeycomb cores constructed according to the presentinvention can be further increased by prestressing the unidirectionallyoriented yarns in the various laminates. Prestressing as such is wellknown and has long been practiced in reinforced concrete constructions.in honeycomb cores the advantages from prestressing the reinforcingmembers have not been generally available. Accordin y, to prestress theyarns they are subjected to tenslon wlu e wrapped over the mandrel andthe tension is maintained until the plastic or resin had time to set upto maintain the yarn in the prestressed condition. A highly desirablefurther increase in the strength-toweight ratio of honeycomb panelsconstructed according to the present invention is thereby obtained.

While one embodiment of the invention has been shown and described, itwill be obvious that other adaptations and modifications can be madewithout departing from the true spirit and scope of the invention.

We claim:

1. A reinforced plastic honeycomb core having a high strength-to-weightratio comprising a plurality of generally sinusoidally formed ribbonsbonded together at spaced apart nodes to define honeycomb cells, eachribbon having in cross section at least one layer of parallel yarnsarranged in side-byside relation only and a binder securing the yarns inthe layer to each other whereby, as compared to a honeycomb coreincluding cross yarns and having deflected longitudinal yarns, anincrease in the honeycomb strength of up to about 40 percent isobtained.

2. A honeycomb core according to claim 1 including a plurality of layersof yarns and means bonding the layers face-toface to each other todefine ribbons constructed of a plurality of layers.

3. A honeycomb core according to claim 2 wherein the yarns in thedifferent layers are angularly inclined with respect to each other.

4. A reinforced honeycomb core having a high strength-toweight ratiocomprising a plurality of generally sinusoidally shaped ribbons bondedtogether at'adjacent nodes and defining honeycomb cells, each ribboncomprising a first layer constructed of a plurality of parallel yarnsarranged side-by-side, extending in like directions and defining a fiatlayer free of cross yarns, a second layer constructed of a sheetmaterial, and means bonding the layers face-to-face to each other.

5. A honeycomb core according to claim 4 wherein the sheet materialcomprises a metallic material.

2. A honeycomb core according to claim 1 including a plurality of layersof yarns and means bonding the layers face-to-face to each other todefine ribbons constructed of a plurality of layers.
 3. A honeycomb coreaccording to claim 2 wherein the yarns in the different layers areangularly inclined with respect to each other.
 4. A reinforced honeycombcore having a high strength-to-weight ratio comprising a plurality ofgenerally sinusoidally shaped ribbons bonded together at adjacent nodesand defining honeycomb cells, each ribbon comprising a first layerconstructed of a plurality of parallel yarns arranged side-by-side,extending in like directions and defining a flat layer free of crossyarns, a second layer constructed of a sheet material, and means bondingthe layers face-to-face to each other.
 5. A honeycomb core according toclaim 4 wherein the sheet material comprises a metallic material.